Leptoquarks are hypothetical particles that would carry information between a generation of quarks and a generation of leptons, thus allowing quarks and leptons to interact. Generally speaking, the quark generation and the lepton generation do not need to be identical. Leptoquarks are color-triplet bosons that carry both lepton and baryon numbers. They are encountered in various extensions of the Standard Model, such as technicolor theories or GUTs based on Pati–Salam model, SU(5) or E6, etc. Their quantum numbers, like spin, (fractional) electric charge and weak isospin, vary among theories.
Leptoquarks, predicted to be nearly as heavy as an atom of lead, could only be created at high energies, and would decay rapidly. A so-called third generation leptoquark, for example, might decay into a bottom quark and a tau lepton. Some theorists proposed that data recorded in experiments at the HERA accelerator at DESY could hint at leptoquarks, which would be a new force that bonds positrons and quarks. Also preons at high energies were considered. More detailed analyses could, however, not confirm these hypotheses. Current best limits on leptoquarks are set by LHC, which has been searching for the first, second, and third generation of leptoquarks and some mixed-generation leptoquarks. For leptoquarks coupling to electrons and up or down quarks, experiments of atomic parity violation and parity-violating electron scattering set the best limits.
Leptoquarks could explain the reason for the three generations of matter. Furthermore, leptoquarks could explain why the same number of quarks and leptons exist and many other similarities between the quark and the lepton sectors. At high energies, at which leptons (which are not affected by the strong force) and quarks (that cannot be separately observed because of the strong force) become one; this could form a more fundamental particle and describe a higher symmetry. There would be three kinds of leptoquarks made of the leptons and quarks of each generation.
In 1997, an excess of events at the HERA accelerator created a stir in the particle physics community, because one possible explanation of the excess was the involvement of leptoquarks. However, later studies performed both at HERA and at the Tevatron with larger samples of data ruled out this possibility for masses of the leptoquark up to around 275–325 GeV. Second generation leptoquarks were also looked for and not found. More recent studies, performed at the LHC, have raised the excluded range to about 1 TeV. For leptoquarks coupling to a neutrino and a quark to be proven to exist, the missing energy in particle collisions attributed to neutrinos would have to be excessively energetic. It is likely that the creation of leptoquarks would mimic the creation of massive quarks.
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